First trimester screening combines an ultrasound and blood tests. It's done during the first trimester of pregnancy, during weeks 11 to 14. It can help find out the risk of the baby having certain birth defects. This includes chromosome defects, such as Down syndrome (trisomy 21), trisomy 18, or trisomy 13.

First trimester screening may include:

• Ultrasound test for fetal nuchal translucency. This test uses ultrasound to look at the back of the baby's neck. It checks for increased fluid or skin thickening. Increased nuchal thickness is associated with an increased risk for chromosome defects and certain birth defects. Nuchal translucency is measured between 11 and 14 weeks gestational age.
• Blood tests. The blood tests measure two substances found in your blood during pregnancy:
  • Pregnancy-associated plasma protein-A. This protein is made by the placenta in early pregnancy. Abnormal levels could mean an increased risk for a chromosome defect.
  • Human chorionic gonadotropin. This hormone is made by the placenta in early pregnancy. Abnormal levels could mean an increased risk for a chromosome defect.
• Cell-free fetal DNA screening. This tests the baby's DNA that is in your blood. It checks for certain abnormal chromosomes. It can also check for defects in the fetal sex chromosomes (X or Y). Cell-free fetal DNA screening does not find structural birth defects, such as spina bifida or defects in the abdominal wall. Cell-free fetal DNA can be done as early as 8 to 9 weeks gestational age.

If first trimester screening is abnormal, you may need more testing for diagnosis. This may include chorionic villus sampling, amniocentesis, or another ultrasound.

First trimester screening can tell if the baby might have a birth defect. The screening test is usually offered during all pregnancies toward the end of the first trimester. Cell-free fetal DNA testing is an alternative to nuchal translucency and maternal serum screening. Some type of screening for the most common chromosomal abnormalities is offered during all pregnancies.

First trimester screening involves an ultrasound and blood tests. These are low-risk tests. But if the tests are not done at the right time during the pregnancy, the results may be wrong. For example, this might happen if your due date was miscalculated. This can cause unnecessary worry and concern for you and your partner.

Cell-free fetal DNA and first trimester combined screening can be done in twin pregnancy but is not available for triplets or more.

First trimester screening is not 100% accurate. It's only a screening test to see if there is an increased risk for a birth defect. It also helps to see if you need more testing or watching during your pregnancy.

False-positive results can show a problem when the baby is actually healthy. False-negative results show a normal result when the baby actually does have a health problem.

You don't need to do anything special to get ready for the ultrasound or blood tests.

First trimester screening generally involves the following tests. They are done during weeks 11 to 14:

• Ultrasound test. An abdominal or transvaginal ultrasound is done to look at the baby. For a transvaginal ultrasound, a small ultrasound transducer is inserted into the vagina.
• Blood test. Blood is drawn from a vein and sent to the lab for analysis. Cell-free fetal DNA blood testing may be done as early as 8 to 9 weeks.

After the tests are done, you will meet with your doctor to review the results. First trimester screening is not 100% accurate, and does not give a diagnosis. If the results are abnormal, your doctor will discuss follow-up testing for a diagnosis. If appropriate, you will be offered genetic testing and counseling.

Before you agree to the test or procedure make sure you know:

• The name of the test or procedure.
• The reason you are having the test or procedure.
• What results to expect and what they mean.
• The risks and benefits of the test or procedure.
• What the possible side effects or complications are.
• When and where you are to have the test or procedure.
• Who will do the test or procedure and what that person's qualifications are.
• What would happen if you did not have the test or procedure.
• Any alternative tests or procedures to think about.
• When and how you will get the results.
• Who to call after the test or procedure if you have questions or problems.
• How much you will have to pay for the test or procedure.